| Liquefied Natural Gas is obtained by natural gas at the atmosphere pressure and at the temperature of -160℃, after a series of treatments such as purification, refrigeration, liquefaction and so on. It is a multi-component mixture, and the change of temperature or component will cause many problems. For example, the stratification and rollover phenomena,the evaporation problems in LNG tank, etc. In order to solve these problems during the storage and transportation processes, it is necessary to know the developing law of the stratification and rollover phenomena, and understand the distribution of temperature field and the evaporation rate in LNG tank.The flow pattern decision criterion of natural convection is established. It is proven by example calculation that the flow pattern of LNG in the storage tank can be decided through this method.Based on the equation of viscous fluid, the mathematical model of LNG stratification and rollover under the condition of disturbed flow is established. Fluent, a software calculating flow and heat transfer of the fluid, is improved to fit the stratification and rollover model of disturbed flow condition, Through analog, it can be seen visually that the developing processes of LNG stratification and rollover can be divided into four states: the state of fixed stratification interface, the state of shifting stratification interface, the state of accelerated shifting stratification interface, and the state of rollover. Through analyses, it can be determined that the stratification of LNG and the flow of thermal boundary layer inside the LNG tank are the basic reason for the rollover of the LNG stratification, and the impact and entrainment effect of central jet stream and the flow of thermal boundary layer around the tank wall are the dominant factors for the damage of the LNG stratification interface. The analog results agree well with the analog modeling results of foreign literature. By virtue of the disturbed flow model, various factors which affect the rollover phenomenon are analyzed, and the measures of preventing LNG stratification are proposed.At home, the pressure and evaporation rate experiment whose test medium is LNG is first done, and its object is closed LNG production tank. The results show that, there exist a non homogeneous temperature field and critical initial fill rate, that is, the temperature of gas phase is higher than the temperature of gas-liquid interface, and the temperature of gas-liquid interface is higher than the host temperature of liquid phase. When the initial fill rate is lower than the critical fill rate, the evaporation rate of a fill rate increases with time. When the initial fill rate is higher than the critical fill rate, the evaporation rate of a fill rate firstly increases with time, and then decreases with time.Based on the experimental results, combined with mass conservation principle and energy conservation principle, the evaporation rate model of three-phase in the closed LNG tank is established. Through example calculation, it can be indicated that this model can be used to predict the variation trend of pressure and evaporation rate in the closed LNG tank .Finally, the model of three-phase is used to analyze various factors which affect evaporation rate of the closed LNG tank. The results show that: there exists a critical initial fill rate within the range of the maximum pressure limited by the storage tank, and there exists an"optimum initial fill rate"in the closed LNG tank. The safety storage time is longest with the optimum initial fill rate. The higher the environment temperature is, and the bigger the thermal conductivity of the LNG tank insulation is, the pressure of the closed LNG tank will increase more quickly, and the safety storage time will become shorter. The higher the nitrogen content is, the higher the evaporation rate will be. These conclusions can provide theoretic criterion for the safety storage and transportation of LNG..
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